Radiotherapeutic apparatus and techniques have now developed to the point where it is likely that some form of tracking or gating system to reduce the effects of tumour motion (primarily due to breathing) will soon be feasible. Such techniques offer significant advantages in terms of the treatment of tumours in the chest and lung area. They require, however, a source of information as to the current location of the tumour, during treatment, in real time. As yet, this is not available, a factor which has the potential to delay their introduction.
Most existing respiration correlation systems use some form of surrogate monitoring system which has a fast response time, in conjunction with a correlation to the internal tumour position. In other words, rather than try to achieve the difficult task of monitoring the actual position of the tumour, either a feature in the patient is identified, or a marker is provided, whose position can more easily be tracked and is correlated with that of the tumour.
Current surrogate monitoring systems include the Varian RPM system, in which an external marker on the surface of the patient is monitored by a camera, the VisionRT camera-based surface tracking system, the Accuray system using a marker vest and cameras, and our use of a pressure sensor in the abdominal compression plate (see WO2008/040379).
Various problems exist with certain of these systems. One problem is how representative these surrogate monitoring systems are of the internal tumour position. For example, some systems generate a correlation between the position of the external marker and the internal tumour position on the 4D planning CT. Some studies show that this correlation is not stable and therefore not valid after a period of time. The use of 4D CT is useful in that it allows the visualisation of soft tissue target and critical structures, but it can not be easily repeated to ensure the correlation continues to be valid. Additionally, 4D CT generally only uses a few breathing cycles per slice, which gives a low statistical confidence in the result.
The Accuray system uses oblique kV planar imaging systems to generate the correlation between marker vest and internal tumour position. The system then periodically checks the correlation and repeats this as required. The change in correlation is particularly a problem for this technique due to the extended treatment time. The use of kV planar imaging systems is good in that it can be easily repeated, but it does not easily allow the visualisation of soft tissue target and critical structures. This system also tends to use very few breathing cycles to determine the correlation, which gives a low statistical confidence in the result.